1. Field of the Invention
The present invention relates to a circuit device, and more particularly, it relates to a circuit device comprising a circuit element.
2. Description of the Background Art
In a circuit device included in an electronic apparatus or the like, the heating value per unit volume has recently been increased due to downsizing, densification and multi-functioning. In recent years, therefore, a metal substrate having high heat radiability has been employed as the substrate for the circuit device, and a circuit element such as an IC (integrated circuit) or an LSI (large-scale integrated circuit) has been mounted on the metal substrate (refer to Japanese Patent Laying-Open No. 7-193347 (1995), for example).
FIG. 16 is a sectional view showing the structure of a conventional circuit device disclosed in the aforementioned Japanese Patent Laying-Open No. 7-193347. Referring to FIG. 16, a wiring structure 102 is formed on a substrate 101 in the conventional circuit device. This wiring structure 102 is formed by stacking a plurality of conductive layers constituting wires (not shown) or the like on an insulating layer. A plurality of thermal via holes 102a are formed on a prescribed region of the wiring structure 102 at prescribed intervals, to pass through the wiring structure 102 in the thickness direction. Conductors 103 formed by the aforementioned conductive layers are charged into the thermal via holes 102a, to completely fill up the thermal via holes 102a. These conductors 103 have planar upper surfaces flush with the upper surface of the wiring structure 102. A metal die pad 104 is arranged on the region of the wiring structure 102 formed with the thermal via holes 102a (conductors 103). A semiconductor element (circuit element) 106 is fixed onto the die pad 104 through an adhesive layer 105. Bonding wires 107 electrically connect the semiconductor element 106 and the wires (not shown) of the wiring structure 102 with each other. An epoxy-based sealing resin layer (not shown) is formed to cover the semiconductor element 106 and the bonding wires 107.
In the conventional circuit device shown in FIG. 16, heat generated in the semiconductor element 106 is transmitted to the wiring structure 102 through the adhesive layer 105 and the die pad 104, and further transmitted to the substrate 101 mainly through the conductors 103 filling up the thermal via holes 102a of the wiring structure 102, to be radiated.
In order to further improve heat radiability of the conventional circuit device, the thermal via holes 102a filled up with the conductors 103 of metal having high thermal conductivity must be densely arranged under the semiconductor element (circuit element) 106.
When the thermal via holes (openings) 102a filled up with the conductors 103 of metal having high thermal conductivity are densely arranged under the semiconductor element (circuit element) 106, however, the metal conductors 103 filling up the thermal via holes 102a thermally expand due to the heat generated in the semiconductor element 106 to apply stress to a portion of the wiring structure 102 around the semiconductor element 106, disadvantageously resulting in separation between the wiring structure 102 and the sealing resin layer. Thus, reliability of the circuit device is disadvantageously reduced.
If a contact hole (opening) (not shown) filled up with a metal conductor 103 connected to a prescribed wire is arranged around the semiconductor element 106, the stress applied to the wiring structure 102 as described above concentrates to this metal conductor 103 filling up the contact hole, to crack the metal constituting the conductor 103. This may result in defective connection of the wire of the circuit device, to also reduce the reliability of the circuit device.